1. Field of the Invention
The present invention is directed to an inductive device, and more particularly to an inductive device having a generally U-shaped core and a straight core defining therebetween a pair of airgaps.
2. Description of the Prior Art
Inductive devices such as inductors and transformers are widely utilized in switching regulators and inverter circuits. For such inductive devices, a ferrite magnetic core with an airgap or airgaps is preferred in order to facilitate inductance adjustment or minimize variations in the inductance, as well as to improve thermal stability as well as DC drooping, i.e., to prevent saturation of the core. As the ferrite cores with the airgaps, there have been proposed in the art an EI-shaped core arrangement with E-shaped and I-shaped core elements, an EE-shaped core arrangement with a pair of opposed E-shaped core elements, or pot-type core arrangement with a barrel core element of E-shaped cross-section. In either of the above types of cores, the core elements are opposed to define therebetween more than one airgap which may cause an external leakage flux. Such external leakage flux will penetrate through adjacent parts such as a casing, capacitors, radiators of semiconductors, and other inductors or transformers to thereby induce heating and noise in an electric appliance or even to considerably alter their electric characteristic and to lower the electrical efficiency thereof.
In order to avoid the above problems, there has been also proposed in the art another core of a modified EE-type in which, as shown in FIG. 13, a pair of E-shaped core halves 1 are combined to form only one airgap 2 between the center legs and a winding 3 is disposed around the center legs to surround the airgap 2. However, with such airgap arrangement, fringing flux appears, as indicated by arrows 4 in the figure, to extend outwardly of the airgap 2 in such a manner as to cross or cut the winding 3 to constitute flux linkage, thereby increasing eddy current losses in the winding 3 and therefore causing excess heating of the winding 3. This is particularly critical in the power inductors for use in switching regulators or inverters operating at a high frequency above 20 kHz to carry a relatively large current.
Also from the viewpoint of obtaining characteristic stability and compact arrangement, the above modified EE-shaped core arrangement having only one airgap between the center legs is found unsatisfactory for the reason that it might incur much inductance variations or larger sizes than the EE-shaped core arrangement where the airgaps are each formed between the outer core legs, and therefore impose a certain limitation on the design of the inductor or the transformers. This is known from the following equation: ##EQU1## where L is the inductance, .mu..sub.o is the permeability of air in vacuum, S.sub.g is the equivalent cross-sectional area of the magnetic circuit at the gap, 1.sub.g is the length of the gap, and N is the number of turns in the winding. In the magnetic circuit of the EE-type core, magnetic flux extends outwardly of the gaps each formed between the outer core legs to give S.sub.g which is greater than for the above modified EE-shaped core. That is, to obtain the same inductance, the above modified EE-shaped core arrangement must have less 1.sub.g than that for the EE-shaped core, and is therefore more susceptible to the influence of dimensional variation, which in turn results in critical variation in the inductance. Alternatively, when N is increased to have the same effect without reducing 1.sub.g, the winding becomes more bulky and fails to meet the compact-size requirement, in addition to that the winding suffers from the correspondingly increased electrical resistance or heating. It is therefore highly demanded for the inductive device to have:
(1) less external leakage flux so as to minimize the influence upon the adjacent parts for reducing noises and improving efficiency;
(2) less winding resistance so as to minimize the amount of the heat dissipated from the winding; and
(3) less susceptible to dimensional variations so as to assure consistent electric inductance and property.
To satisfy the above requirements, it may be effective to present a unique core configuration comprising a U-shaped core and a straight core which is disposed between the opposed legs of the U-shaped core to define airgaps at the respective ends of the straight core. With this core configuration and a winding disposed around the straight core, it is readily possible to greatly reduce the external leakage flux emanating from the ends of the straight core. Also, with this core configuration, the air gap is allowed to be spaced away from the winding such that it sees no substantial flux linkage to thereby reduce the amount of heating at the winding.
The like core configuration is seen in Japanese Utility Model Publication (KOKOKU) No. 53-30992, although the publication does not teach to reduce the fringing flux or to improve electromagnetic characteristics, but to disclose a useful layout for supporting the windings to the core by adhesives. As schematically shown in FIG. 14, the core of the Japanese publication includes a U-shaped core 10 with a pair of opposed legs 12 each having an inward projection 13 at its free end. A straight core 15 is disposed between the inward projections 13 in alignment therewith to define a pair of airgaps 16 between the straight core 15 and the inward projections 13. Each of the airgaps 16 is surrounded by each one of windings 18. Although this core configuration is effective to reduce the external leakage flux, it still suffers from the flux linkage at each of the airgaps, which eventually results in the undesired heating at the winding. Also, due to the presence of the inward projection 13, the U-shaped core 10 sees, in addition to two flux converging points P.sub.1 at the inner ends of the legs 12, two more flux converging points P.sub.2 at the corners between the projections 13 and the legs 12. Since the flux converging is disadvantageous when reducing magnetic saturation, losses, heating of the winding, the number of the flux converging points should be kept at a minimum. In this respect, the above Japanese publication is found insufficient and unsatisfactory. Further, at the assembly of fixing the straight core 15 to the U-shaped Core 10, it is often required to adjust the relative position of the straight core 15 to the U-shaped core 10 in order to compensate for dimensional variations thereof and to give a desired inductance. The adjustment can be made by shifting the straight core 15 along the length of the core legs 12 to vary the magnetic resistance. However, with the core configuration of FIG. 14, the shifting of the straight core 15 from the aligned position with the inward projections 13 would cause an abrupt change in the gap distance, thereby causing an correspondingly abrupt change in the inductance, as indicated by dotted line in FIG. 15, therefore failing to assure precise inductance adjustment at the assembly of the core.